International Astronomical Union IERS / International VLBI Service

The International Celestial Reference Frame

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Adopted by the International Astronomical Union as the fundamental celestial reference frame, replacing the FK5 optical frame as of 1 January 1998.

The definition and the creation of the International Celestial Reference Frame (ICRF) was a joint cooperative effort of a sub-group of the International Astronomical Union (IAU) Working Group on Reference Frames which was formed expressly for the purpose of creating the definitive catalog of extragalactic radio source positions using the best data and methods available at the time the work was done. The ICRF is the realization of the International Celestial Reference System (ICRS) at radio wavelengths. Monitoring the ICRS and maintaining its current realization, the ICRF, is the responsibility of the ICRS Product Center of the International Earth Rotation and Reference Systems Service (IERS). .

Definition of the ICRF

"The International Celestial Reference Frame as Realized by Very Long Baseline Interferometry,"
1998, Astronomical Journal, 116, 516
ABSTRACT

A quasi-inertial reference frame is defined based on the radio positions of 212 extragalactic sources distributed over the entire sky. The positional accuracy of these sources is better than about 1 mas in both coordinates. The radio positions are based upon a general solution for all applicable dual-frequency 2.3 and 8.4 GHz Mark III very long baseline interferometry data available through the middle of 1995, consisting of 1.6 million pairs of group delay and phase delay rate observations. Positions and details are also given for an additional 396 objects that either need further observation or are currently unsuitable for the definition of a high-accuracy reference frame. The final orientation of the frame axes has been obtained by a rotation of the positions into the system of the International Celestial Reference System and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy. The resulting International Celestial Reference Frame has been adopted by the International Astronomical Union as the fundamental celestial reference frame, replacing the FK5 optical frame as of 1998 January 1.


Maintenance of the ICRF

The International Astronomical Union (IAU) has charged the International Earth Rotation and Reference Systems Service (IERS) with the responsibility of monitoring the International Celestial Reference System (ICRS) and maintaining its current realization, the International Celestial Reference Frame (ICRF). Starting in 2001, these activities are run jointly by the IERS ICRS Product Center (a collaboration between the l'Observatoire de Paris and the U.S. Naval Observatory) of the IERS and the International VLBI Service for Geodesy and Astrometry (IVS), in coordination with the IAU Working Group on Reference Systems.

First Extension of the ICRF

"First extension of the ICRF, ICRF-Ext.1,"
1998 IERS Annual report, 1999: Chapter VI, D. Gambis (ed), Observatoire de Paris, p. 87-114.
ABSTRACT

The ICRF maintenance and extension catalogue ICRF-Ext.1 contains 59 new sources that have been observed since July 1995 and provides refined positions of candidate sources of the ICRF from additional observations.


Second Extension of the ICRF

"The Second Extension of the International Celestial Reference Frame: ICRF-Ext.2,"
2004, Astronomical Journal, 127, 3587
ABSTRACT

We use Very Long Baseline Interferometry data obtained between mid-1995 and the end of 2002 May together with older data to extend and revise the International Celestial Reference Frame. Revised positions of ICRF candidate and "other" sources, based on inclusion of the additional data, are presented. Positions, in the frame of the ICRF, for an additional 109 new sources are also presented. All but four of the new sources are located north of declination equal to -30 degrees. Positions of the ICRF defining sources remain unchanged. We present a summary of current astrometric/geodetic observing programs and discuss the evolution and future of the ICRF.


Last modified: Tue Dec 19 08:42:48 EST 2006
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